DE 10 2004 021 047 B3 discloses a method for comparing an image with at least one reference image using a system which comprises at least one image capturing unit and an image processing system, equipped with a logic unit, for evaluating image data from the image capturing unit. An image captured by the image capturing unit is compared with the at least one reference image, with the logic unit carrying out the image comparison. The image comparison evaluates the quality of printed matter produced by a printing machine. The logic unit evaluates the quality of the printed matter with respect to a multiplicity of criteria while a production process is running on the printing machine, with the evaluation of the quality of the printed matter produced by the printing machine being divided into at least two process units. The image capturing unit transmits the image data, captured by its image sensor, as digital data to the logic circuit.
DE 43 21 177 A1 discloses a method for inspecting an image on at least one printed product produced in a printing machine. All relevant digital image data for the printed product are compared in a computer device through a desired value/actual value comparison with desired data for an o.k. image. A method of this type will necessarily result in an error message if, at the time the image data are recorded, the printed product is itself spatially variant or has at least one element which is spatially variant with regard to a reference point which is fixed in relation to said printed product.
EP 2017 082 is directed to a method involving detecting a periodic status signal of a marking element and optically detecting a portion of a continuous web, such as a printing web, and converting into data. The data is stored to synchronize a respective origin of an exact repeating pattern with subsequent repeating pattern and/or similarly formed printing web. A grey vector and/or a color vector are estimated by the optical detection of the printing web.
In security printing in particular, but also in premium quality package printing, printed products are produced, e.g. banknotes or security documents or folding boxes, which have at least one element which is spatially variant with regard to a fixed reference point, with said reference point being designed as fixed, i.e. spatially fixed, in relation to the printed product or the copy of said printed product in question. A spatially variant element of this type may be a shiny strip, in particular a pearlescent strip or Iriodin strip, for example, incorporated into the printed product in question and/or into a copy of said printed product. When the printed product in question or the copy of said printed product is tilted against a light source, an Iriodin strip will appear as a shiny strip, the color of which changes, e.g. from bright yellow to golden yellow, and then disappears. Shiny strips are used on banknotes in particular as a security feature. In a series comprising multiple copies of printed products that are each provided with a shiny strip, e.g. when a multiplicity of banknotes are produced as copies in security printing, a shiny strip of this type can be spatially variant with regard to a fixed reference point, e.g. an edge of the copy of the printed product in question or the center thereof, from one copy to the next copy of the printed product in question, depending on the production process. This means that, in an inspection of this series of copies of said printed product, the position of the shiny strip may change at least slightly from one copy of the printed product to another copy in the same series of the printed product, and may deviate from an anticipated position, wherein this deviation may be irregular and without a preferred direction. In an inspection using a spatially fixed image capturing device and a conventional desired value/actual value comparison, in which current image data are compared with previously established statistical reference image data, e.g. taken from a prepress, this variability in the alignment and/or positioning of the spatially variant element, designed, e.g. as a shiny strip, will result in an error message.